In rotary drilling of subterranean wells numerous functions and characteristics are expected of a drilling fluid. A drilling fluid should circulate throughout the well and carry cuttings from beneath the bit, transport the cuttings up the annulus, and allow their separation at the surface. At the same time, the drilling fluid is expected to cool and clean the drill bit, reduce friction between the drill string and the sides of the hole, and maintain stability in the borehole's uncased sections. The drilling fluid should also form a thin, low-permeability filter cake that seals openings in formations penetrated by the bit and act to reduce the unwanted influx of formation fluids from permeable rocks.
Drilling fluids are typically classified according to their base material or primary continuous phase. Due to environmental concerns, focus has increased on water-based fluids. Three types of solids are usually found in water-base drilling fluids: (1) clays and organic colloids added to provide necessary viscosity and filtration properties, (2) heavy minerals whose function is to increase the drilling fluid's density, and (3) formation solids that become dispersed in the drilling fluid during the drilling operation.
The formation solids that become dispersed in a drilling fluid are typically the cuttings produced by the drill bit's action and the solids produced by borehole instability. Where the formation solids are clay minerals that swell, the presence of such solids in the drilling fluid can greatly increase drilling time and costs. The overall increase in bulk volume accompanying clay swelling impedes removal of cuttings from beneath the drill bit, increases friction between the drill string and the sides of the borehole, and inhibits formation of the thin filter cake that seals formations. Clay swelling can also create other drilling problems such as loss of circulation or pipe sticking that can slow drilling and increase the drilling costs.
Clay swelling is a phenomenon in which water molecules surround a clay crystal structure and position themselves to increase the structure's c-spacing. Two types of swelling can occur. Examples of clay swelling include surface hydration and osmotic swelling.
Surface hydration is a type of swelling in which water molecules are adsorbed on crystal surfaces. Hydrogen bonding holds a layer of water molecules to the oxygen atoms exposed on the crystal surfaces. Subsequent layers of water molecules then line up to form a quasi-crystalline structure between unit layers which results in an increased c-spacing. All types of clays swell in this manner.
Osmotic swelling is a type of swelling where the concentration of cations between unit layers in a clay mineral is higher than the cation concentration in the surrounding water, water is drawn between the unit layers and the c-spacing is increased. Osmotic swelling results in larger overall volume increases than surface hydration. However, only certain clays, like sodium montmorillonite, swell in this manner.
Although a number of compounds are known for their effectiveness in inhibiting reactive shale formations, several factors affect the practicality of using swelling inhibitor additives in drilling fluids. First, the inhibitor must be compatible with the other drilling fluid components. The driller of subterranean wells must be able to control the rheological properties of drilling fluids by using additives such as bentonite, anionic polymers and weighting agents. Thus, drilling fluid additives should also provide desirable results but should not inhibit the desired performance of other additives. However, many swelling inhibitors will react with other drilling fluid components, resulting in severe flocculation or precipitation.
Second, current drilling fluid components must be environmentally acceptable. As drilling operations impact on plant and animal life, drilling fluid additives should have low toxicity levels and should be easy to handle and to use to minimize the dangers of environmental pollution and harm to personnel. Moreover, in the oil and gas industry today, it is desirable that additives work both onshore and offshore and in fresh and salt water environments.